This invention relates to an aluminium processing apparatus for separating aluminium from a mixture of aluminium and aluminium dross, and to a related process for separating aluminium from a mixture of aluminium and aluminium dross. In particular, the invention relates to a process for recycling aluminium from dross produced during an aluminium melting processes, and to an aluminium processing apparatus for carrying out that recycling process.
When aluminium is melted eg. for manufacture of extrusions, ingots and billets, because of the influence of oxygen from environmental air on the aluminium and the existence of impurities, particularly oxides, nitrides and carbides, in the molten aluminium, a layer of sludge, also known as dross rises to the surface of the molten aluminium. This layer of dross has to be removed from the molten aluminium before the molten aluminium can be cast. This is done by the use of a ladle in a rather crude process, known as skimming, in which the ladle is dragged across the top of the molten aluminium and the dross is scraped into a suitable receptacle. During the skimming process, as well as removing sludge including oxides and other impurities, pure aluminium is also removed. The quantity of pure aluminium removed depends on the depth to which the ladle is inserted in the aluminium to ensure removal of all the dross and to a large extent depends on the skill of the furnace worker handling the ladle. However, typically 30 to 60% of the mixture/dross, by weight is aluminium.
The term dross, as used herein, refers to the impurities such as oxides which float to the surface of the molten aluminium, but the term is also used in the art to refer to the mixture of aluminium and the impurities.
Because of the amount of aluminium in the mixture, it is obviously desirable to remove as much aluminium from the dross/aluminium mixture as possible. Almost all recycling is currently carried out using a process known as rotary salt furnace processing. In that process, the dross containing pure aluminium is first allowed to cool. The longer the aluminium spends hot, the more oxidation occurs and less aluminium is recovered in the recycling process, so often cooling is encouraged and accelerated. In some cases some initial separation of aluminium from the mixture is first carried out by one of two rather inefficient devices know as drain pans and dross presses, respectively. In the former the mixture is allowed to sit while molten and some of the aluminium will sink to, and agglomerate in, the bottom of the pan. In the latter, the mixture is compressed and the aluminium droplets tend to stick together. U.S. Pat. No. 5,788,918 to Bramely discloses one example of a dross press. These processes are inefficient and have to be followed by rotary salt processing or other methods of external dross processing. Because the mixture is kept hot longer for the drain pan or dross press process, the recovery rate in the subsequent rotary salt process is reduced, so drain pans and dross presses are generally not commercially viable, and are not often used.
Recycling is not generally done at the furnace, but is usually is done by specialist metal-recycling companies. In the rotary salt recycling process, the dross is heated and remelted and various salts and fluxes are added in order to separate the aluminium from the oxides and other impurities. While the process is highly efficient in terms of the quantity of aluminium removed from the dross, removing approximately 85% of the available aluminium, the waste products from the recycling process, ie the mixture of salts and the oxides, is unpleasant, very environmentally unfriendly, and difficult to dispose of safely. Further, the process requires the transporting of the dross to the recycler in trucks or the like which is also undesirable from an environmental point of view, and inefficient in terms of fuel. Also, the dross has to be remelted in order to extract the aluminium in the recycling process which requires a substantial amount of energy. There are some smelting plants which have their own rotary salt recycling furnace, however, the process of cooling and transporting the cooled mixture to the furnace remains the same, although savings are made in total transport costs.
Proposals have been made for separating aluminium from dross in the past. GB 1533696 and U.S. Pat. No. 3,689,049 disclose two different devices for separating aluminium from dross. Neither device has had any commercial success, perhaps because they are over-complicated and too unreliable for the extreme environment in which they have to operate.
The present inventor has also invented an apparatus and process, disclosed in AU 56260/98, and Greek patent No 97-0100106 that provides a simpler and more cost effective method of recycling aluminium from dross. The present invention is directed to improvements to the apparatus earlier developed by the inventor.
In a first aspect of the present invention there is provided an aluminium processing apparatus for separating molten aluminium from a mixture of molten aluminium and aluminium dross comprising:
a table for supporting an insulated crucible, the crucible having an open top for containing the mixture of molten aluminium and aluminium dross;
vibration means for vibrating the crucible when supported on the table;
a frame adapted to support a paddle means, means for rotating the paddle means and means for relatively lowering the paddle means into the mixture in the crucible for stirring the mixture with the paddle means, the paddle means comprising a plurality of tines having a generally triangular cross section with a ridge of the triangular cross section being uppermost in use when lowered into the crucible;
a shroud adapted to cover the top of the crucible and maintain an oxygen reduced atmosphere in the crucible; and
a screening means comprising walls mounted to the frame which substantially surround the crucible when it is supported by the table, the screening means having at least a portion thereof movable between at least a first retracted position to allow access to the crucible and at least a second position where it prevents access to the crucible; wherein simultaneous vibration of the crucible by the vibration means and stirring of the mixture by the tines is enabled with the tines moving through the dross in a shearing action mixing and tumbling the mixture and causing the droplets of aluminium to coalesce.
The tines are removably attached to the rotatable plate for cleaning or repair.
The shroud may be formed from a heat insulating material and may be located adjacent the rotatable means to be lowered with the paddle means to cover the crucible in use.
The screening means can be removably mounted to the frame. In this embodiment, the movable portion may comprise a door that is retractable to allow access to the table of the apparatus. The door can be adapted to move upwardly, sidewards or downwardly to provide a portal for access to the table. The door can be slidably movable relative to the remainder of the screening means. In another embodiment, the door can be pivotally mounted to the wall. When the door is fully retracted, the portal must be of sufficient size to allow the crucible to pass through the portal.
In another embodiment, the entire screening means can be movable between the first retracted position and the second position. In this embodiment, the screening means can extend downwardly from the lowering means for the paddle means and is so movable relative to the table and crucible in concert with the operation of the lowering means.
The screening means is preferably adapted to prevent or at least ameliorate the escape of heat from the vicinity of the crucible. By preventing the escape of heat, the screening means serves to protect workers working near the apparatus. It also serves to slow the rate of cooling of the crucible and its contents which is desirable. The screening means can also preferably be adapted to prevent or at least substantially prevent the escape of dust and flame from the vicinity of the crucible. The screening means can also be adapted to safeguard workers working in the vicinity of the apparatus from explosions of aluminium and dross from the crucible.
The screening means is preferably formed from a multi-layer structure, comprising at least an inner layer and an outer layer. The inner and outer layer can be fabricated from a suitable metal or other material, including refractory or cementitious materials.
The crucible may be made of a refractory material having a metallic outer skin spaced from the refractory material by a layer of insulation. The crucible can be provided with a covering means that at least partially covers the upper opening of the crucible. The cover can be retractable or otherwise removable from the crucible when the paddle or stirring means is relatively lowered into the crucible. If desired, the cover can be adapted to surround the shaft of the paddle or stirring means while the paddle or stirring means is stirring the crucible""s contents.
The table may be mounted on springs to assist in vibrating the crucible. Vibration of the table and crucible may be achieved by the mounting of at least one ultrasonic transducer, electric vibrator, mechanical vibrator or a combination of one or more of these transducers to the crucible and/or table. The table can be vibrated such that it oscillates in a horizontal axis, a vertical axis or in both axes.
The frame may consist of two or more upright pillars. A cross-beam may be supported by the frame and that cross-beam may carry the rotating means. The shaft for the blades may depend down from the cross-beam with the blades mounted on the lower end of that shaft.
The apparatus can include one or more exhaust pipes that allow gaseous product from the process performed by the apparatus to be vented to atmosphere outside the screening means. The exhaust pipes can incorporate bagging systems that collect the exhaust gaseous product for later disposal. The bagging systems can be adapted to also or instead extract dust from the gaseous product. The exhaust systems can also include filtering or scrubbing means that clean or substantially clean the gaseous product from the process of harmful or potentially harmful constituents.
The apparatus can include a weighing means, such as an electronic or mechanical scale, that allows measurement of the weight of the crucible and its contents while the crucible is sitting on the table. The apparatus can also include a control means. The control means is preferably adapted to allow an operator to operate all features of the apparatus from a central location. In a further preferred embodiment, the control means can be automated such that the apparatus runs normally without operator input.
The control means may also control the sequence and rate of the vibrations.
While the size of the crucible containing the dross is not important, typically the crucible should be large enough to carry between 800 kilograms to 1.5 tonne of dross. The process takes approximately 10 minutes and removes up to 95% of the aluminium in the dross, hence the present invention can separate approximately 1000 kilograms of aluminium from dross every 4-10 minutes.
Typically, the crucible will include plugholes which are plugged prior to carrying out the separation process with for example a cone or a sand and clay solution, and are unplugged to drain the crucible into a draining pan.
In yet a further aspect, the invention involves a method for separating molten aluminium from a mixture of aluminium and aluminium dross comprising the steps of:
removing the mixture from a furnace containing molten aluminium;
transferring the hot dross to an insulated crucible;
transferring the crucible to a table means;
inserting a paddle means into the mixture in the crucible;
relatively lowering a screening means such that it substantially surrounds the crucible;
stirring the mixture with the paddle means and simultaneously vibrating the table and crucible to cause the oxide skin on aluminium droplets in the dross to break, and to cause the aluminium droplets to coalesce, such that the droplets enlarge in volume and in weight and tend to sink to the lower part of the crucible; and
removing aluminium from the lower part of the crucible.
In still yet a further aspect, the invention involves a method of separating molten aluminium from a mixture of aluminium and aluminium dross comprising the steps of:
removing the mixture from a furnace containing molten aluminium;
transferring the hot dross to an insulated crucible;
transferring the crucible to a table means;
inserting a paddle means into the mixture in the crucible, the paddle means comprising a rotatable means and a plurality of tines depending from the rotatable means said tines having a generally triangular cross section, with a tip or ridge of the triangular cross section being uppermost in use when the tines are lowered into the crucible;
relatively lowering a screening means such that it substantially surrounds the crucible;
stirring the mixture with the tines and simultaneously vibrating the table and crucible with the tines moving through the dross in a shearing action mixing and tumbling the mixture to cause the oxide skin on aluminium droplets in the dross to break, and to cause the aluminium droplets to coalesce such that the droplets enlarge in volume and in weight and tend to sink to the lower part of the crucible; and
removing aluminium from the lower part of the crucible.
The pure aluminium can then be either cast as an ingot or recycled straight back into the furnace containing molten aluminium or drained into a crucible.
The process is carried out without substantially cooling the dross, preferably at a temperature of about 750xc2x0 C.
In a yet further aspect the invention provides an apparatus for separating molten aluminium from a mixture of molten aluminium and aluminium dross including:
a table for supporting an insulated crucible containing the mixture of molten aluminium and aluminium dross;
at least one heating means such as a gas burner of the like adapted to either preheat the paddles or heat the contents of the crucible or heat both.
a frame adapted to support a paddle means or stirring means, means for rotating the paddle means and means for relatively lowering the same into the mixture in the crucible for stirring the mixture with the paddle means enabling simultaneous vibration of the crucible and stirring of the mixture.
The gas burners of the like can be used to maintain the heat in the contents of the crucible during the stirring process. In contrast with prior art devices such as dross presses maintaining heat during the separation process has been found to be desirable.